We have examined the virus-specific RNA and proteins in avian sarcoma virus transformed cells. Using sera from tumor-bearing animals and translation of viral RNA in cell-free extracts we have identified the product of the avian sarcoma virus transforming gene which appears to be a phosphoprotein which is associated with a protein kinase activity. We propose to carry out additional chemical and functional structures of this polypeptide with the aim of understanding how it may mediate cell transformation.